So READINGS IN EVOLUTION, GENETICS, AND EUGEN^CS 



Elephas. — In this genus are included a number of extinct forms 

 (the mammoths) from three or four continents, and the n\dng ele- 

 phants. The extinct forms, though called mammoths, were not large 

 animals, being no larger than the Indian elephant of today, and not 

 so large as the living African species. Some of the features of the 

 elephants, their size, the short neck, the long proboscis, and the hea\-y 

 tusks are matters of common observation. The skull is very high 

 and short (Fig. 5, A'). The height is due chiefly to the development 

 of cancellate bone, not to the enlargement of the brain, which is still 

 quite small. As stated above, the high skull affords the necessary 

 leverage for the muscles that support the weight of the tusks. The 

 molar teeth are distinctly grinding teeth (Fig. 5, A). Each tooth 

 bears a number of transverse ridges, about ten in the African elephant 

 and two dozen or more in the Indian species. These ridges are worn 

 down by the che-wing of harsh food, so that the upper surface displays 

 a number of flattened tubular plates of enamel inclosing dentine and 

 bound together by cement. A tooth is completely worn out by use, 

 and is replaced by another. The method of replacement, however, 

 is peculiar. WTiile the tusks (incisors) are of two sets, one following 

 the other like milk and permanent teeth of other mammals, the 

 grinders succeed one another in continuous fashion. There are never 

 more than two \'isible grinders on each side of each jaw. As they 

 wear out they move forw^ard in the jaw, and are replaced by new teeth 

 appearing behind. New molars thus enter at intervals of two to four 

 years in young elephants, and at intervals of 15 to 30 years in later 

 life. If an elephant lives long enough (60 years or more) it develops 

 a total of 28 teeth, including tusks, but has not more than ten (often 

 less) at any one time. 



Correlated with the nature of the teeth of the elephants are their 

 food and che'sving habits. WTiereas the ancestral forms whose molars 

 bore prominent elevations lived on twigs and tender herbage which 

 they crushed in mastication, the mammoths with their flattened tooth 

 surfaces devoured grasses, sedges, and other harsh vegetation which 

 they ground with lateral motion of the teeth upon one another. In 

 this respect modem elephants are like the mammoths. 



In the changes described above is found one of the most beautiful 

 and best established evolutionary series with which the palaeontolo- 

 gist is acquainted. Only a few others equal or approach it in clearness 

 and completeness. 



t 



li 



